The invention relates to the isolation of stone cells from fruit.
Stone Cells are hard, granular materials predominantly found in pears and quince, as well as in other fruits such as sapota and guava. The stone cells cause the interior of these fruits to have a gritty texture. In botanical terminology, stone cells are referred to as sclereids, and are described as substantially lignified plant structures, which are part of the sclerenchyma of plant tissue.
There are several varieties of sclereids. Many are wiry, thin, elongated and often-branched structures, appearing somewhat like short veins. These vein-like sclereids provide physical strength to certain plant parts such as leaves, stems, fruits, etc. Other sclereids turn into lignified and dense shells such as those surrounding nuts, for example, walnut shells, almond shells, and pecan shells. The shells of pits of fleshy fruits, such as those of peaches, apricots, plums, and the like are also typical examples of sclereid material. Still others, specifically referred to as stone cells (and also known as brachysclereids), possess thick, lignified walls, and are granular, small, and dimensionally isodiametric. These sclereids are characterized by aspect ratios (length divided by width) in the range of 1 to 4, most typically about 1 to 2.
Stone cells are naturally present in the form of clusters of primary stone cells. The primary stone cells are generally very small particles (about 20 to 50 microns) and are usually agglomerated and lightly “fused” into isodiametric clusters 100 to 800 microns).
Stone cells are naturally attached to the fibrous pulp of the fruit. Stone cells are known to be indigestible and not pleasant to the palate when eaten. For those reasons, efforts have been made to remove stone cells from pulp so that the pulp may be added to fruit juice. For example, Japanese Patent Application No. 55102379, credited to Yamane, discloses a purely mechanical process for removing stone cells from pear juice. The method includes vigorously stirring the fruit juice, stopping the agitation, and allowing the stone cells to settle to the bottom of the tank. The juice product is then withdrawn from the upper portion of the tank, and the stone cells are taken out at the bottom.
Enzymes have also been proposed in order to aid in the removal of stone cells from fruit pulp. For example, U.S. Pat. No. 3,860,726, issued to Yamane, teaches using a peptidase to accelerate the separation of stone cells from pulp-like material. Yamane teaches use of the peptidase provides good chemical separation between the pulp and the stone cells and thus eliminates the need for common exfoliating and separating devices that cause a decrease in the viscosity of the puree. Yamane also discloses that a pectinase could be used for the same purpose; however, the pectinase undesirably causes a large decrease in the viscosity of the crude puree after the pectinase treatment. The objective of Yamane is to provide a puree that is stone cell-free, has a similar viscosity to the puree before stone cell removal, and as such is suitable for producing a fruit nectar. Accordingly, Yamane does not disclose any methods for treating the separated stone cells which are presumably just discarded as waste.
Stone cells are sometimes segregated (mostly unintentionally) during the processing of various plant materials, but such impure stone cells are heavily contaminated with pulp-like fibers, and thus the gritty and pulpy mass is treated as an undesirable by-product. Therefore, this “sludge” is merged into the regular waste stream of the process, e.g., in the processing of some fruits, and the making of paper products. Sometimes sclereids are separated on a large scale by a wet centrifugation method (e.g., a hydrocyclone) for improving a given product, and/or for semi-quantifying sclereid contents in fruit sauces, juices, and pulps in papermaking. Again, since there is no commercial use for these sclereids, they are disposed of as waste.
Until now, the only commercial uses of sclereid-derived products, known by the present inventors, are those arising from the grinding up of the hard sclereids of various nut shells to be used as abrading materials in “sand blasting” operations, and as an additive in some cosmetic compositions, such as exfoliating creams. Known techniques for “handling” unprocessed/unsegregated stone cells for viewing under the microscope do not actually isolate them. There are numerous descriptions in the botanical literature on how to visualize stone cells on a microscope stage (using lignin-specific stains), but these methods do not lead to an actual, deliberate isolation of the individual stone cells by separating the stone cells from its surrounding fleshy pulp.
The present inventors have discovered that isolated stone cells may find utility in commercial applications. Further the present inventors, have developed a method that provides virtually complete removal of the pulp from the stone cells. The inventive method provides free-flowing stone cells that slide past each other, allowing the dry mass of stone cells to flow smoothly like fine sand. The sand like stone cell material can be used in several applications.